room-temperature semiconductor gas sensor based on nonstoichiometric tungsten oxide nanorod film

Room-temperature semiconductor gas sensor based on nonstoichiometric tungsten oxide nanorod film Yong Shin Kim,a兲Seung-Chul Ha, Kyuwon Kim,b兲 Haesik Yang,c兲 Sung-Yool Choi, and Youn Tae

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Room-temperature semiconductor gas sensor based on nonstoichiometric tungsten oxide nanorod film

Yong Shin Kim,a兲Seung-Chul Ha, Kyuwon Kim,b兲 Haesik Yang,c兲

Sung-Yool Choi, and Youn Tae Kim

Electronics and Telecommunications Research Institute, Daejeon 305-350, Republic of Korea

Joon T Park

Department of Chemistry and School of Molecular Science, Korea Advanced Institute of Science

and Technology, Daejeon 305-701, Republic of Korea

Chang Hoon Lee, Jiyoung Choi, Jungsun Paek, and Kwangyeol Leea兲

Department of Chemistry and Center for Electro- and Photo-Responsive Molecules, Korea University,

Seoul 136-701, Republic of Korea

共Received 16 August 2004; accepted 5 April 2005; published online 19 May 2005兲

Porous tungsten oxide films were deposited onto a sensor substrate with a Si bulk-micromachined

hotplate, by drop-coating isopropyl alcohol solution of highly crystalline tungsten oxide共WO2.72兲

nanorods with average 75 nm length and 4 nm diameter The temperature-dependent gas sensing

characteristics of the films have been investigated over the mild temperature range from

20 to 250 ° C While the sensing responses for ammonia vapor showed increase in electrical

conductivity at temperatures above 150 ° C as expected for n-type metal oxide sensors, they

exhibited the opposite behavior of unusual conductivity decrease below 100 ° C Superb sensing

ability of the sensors at room temperature in conjunction with their anomalous conductivity

behavior might be attributed to unique nanostructural features of very thin, nonstoichiometric

Miniaturized solid-state chemical sensors have played an

important role in chemical process controlling, pollutant

monitoring, personal safety, medical diagnosis, and sensor

networks In particular, metal-oxide-semiconductor 共MOS兲

sensors are very promising due to their high sensitivity, small

dimensions, low cost, and good compatibility with the

fabri-cation process for microelectronic devices They operate on

the basis of the modification of electrical conductivity of

metal oxide layers, resulting from the interactions between

ionosorbed moieties such as O2−, O−, and O2−species and gas

molecules to be detected In conventional MOS sensors

con-sisting of polycrystalline metal oxide particles typically with

the average size of 10 nm– 1␮m, only the species adsorbed

near grain boundaries are operative in modifying the

electri-cal transport properties and therefore the gas-sensing ability

has been greatly hampered by low surface-to-volume ratio

One-dimensional共1D兲 nanostructures with high

surface-to-volume ratio and small grain size have attracted much

current attention as candidate materials for solid-state gas

sensors Recently, nanosensors fabricated by using individual

carbon nanotube,1 SnO2 nanoribbon,2 or nanowire,3 and

In2O3nanowire4exhibited properties such as high sensitivity,

fast response time, and room temperature operation, which

are unattainable by the conventional materials The major

drawback, however, remains due to difficulties in mass

pro-duction of sensors based on individual 1D nanostructures

Alternative promising approach is to prepare porous

struc-tures from highly crystalline and phase-pure 1D nanomateri-als These films could be easily fabricated by wet processes and their sensing characteristics were found to be superior to those of conventional MOS.5 In addition, the compatibility

of wet processes with microelectronic fabrication offers par-ticular opportunities for development of inexpensive sensor systems in the cost-conscious gas sensor market

Nanosized tungsten oxide particles have been found use-ful in fabricating gas sensors for the detection of nitrogen oxides,6ammonia,7and hydrogen sulfide.8Current research, however, has focused on the use of polycrystalline tungsten oxide systems for these applications, and thus important sen-sor requirements such as high sensitivity and reproducibility, which can be obtained only by using size-controlled pure nanomaterials, have not been accomplished We have re-cently reported a single step, large scale preparation of single crystalline, size-controlled tungsten oxide nanorods,9 thus providing a singular research opportunity for tungsten oxide-based gas sensor development In this letter, we report fab-rication of a MOS gas sensor, which entails drop coating of

WO2.72 nanorod solution on the Si bulk-micromachined membrane equipped with a hotplate for temperature regula-tion, as well as its unusual gas sensing behavior dependent

on operation temperatures

Sensor substrates were fabricated on Si wafers by using microelectromechanical system 共MEMS兲 and silicon tech-nology, as previously reported in detail.10They have a square membrane共2⫻2 mm2兲 embedded with interdigitated detec-tion electrodes and a Pt microheater, which enables tempera-ture regulation of a sensing layer under minimized power consumption Figures 1共a兲 and 1共b兲 show a cross-sectional schematic diagram and a plane-view optical-microscope im-age of the sensor device, respectively The well structure with the membrane was fabricated by the anisotropic wet

a 兲Authors to whom correspondence should be addressed; electronic mail:

yongshin@etri.re.kr; kylee1@korea.ac.kr

b 兲Current address: Korea Research Institute of Standards and Science,

Dae-jeon 305-600, Republic of Korea.

c 兲Current address: Department of Chemistry, Pusan National University,

Pu-san 609-735, Republic of Korea.

Downloaded 11 Aug 2005 to 143.248.37.84 Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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etching of bulk Si using 5 wt % tetra-methyl ammonium

hydroxide 共TMAH兲 solution The interdigitated detection

electrodes have 100␮m width and 300␮m spacing The

heater line 共25␮m width and spacing兲 simultaneously acts

as a temperature-measuring resistor The sensing layer

dis-played as a circle-shape blot at center of Fig 1共b兲 was

formed by using the tungsten oxide nanorods with 4 nm

di-ameter and aspect ratio of⬃20, which were synthesized

ac-cording to our colloid-based synthetic approach.9The

isopro-pyl alcohol solution of tungsten oxide nanorods prepared by

ultrasonic treatment was dispensed onto the membrane, and

then the membrane was dried at 100 ° C under vacuum for

10 h The well structure allows dropped solution to be placed

reproducibly in a specific, well-constrained area

Figure 1共c兲 shows a scanning electron microscope

共SEM, Philips XL-30兲 image of a tungsten oxide nanorods

film The film was fabricated under the same experimental

conditions on a silicon substrate instead of on the Si-based

membrane due to the difficulties in sample handing and

availability The resulting porous film consists of randomly

arranged linear aggregates which are formed by parallel

alignment of individual WO2.72nanorods The x-ray

diffrac-tion 共XRD, Rigaku D/MAX-RC兲 patterns recorded for this

film demonstrated the 共010兲 peak of monoclinic

WO2.72共W18O49兲, assigned to the growth direction of the

rods,9in addition to broad background peaks resulting from

the small nanorod size of 75 nm length and 4 nm width

Furthermore, the Raman shift bands observed at 264, 325,

709, and 805 cm−1 give further evidence to the monoclinic

tungsten oxide structure.11The constituents of the films were

also analyzed by XPS共VG Scientific, ESCALAB 200R兲 and

found to be composed predominantly of tungsten and oxygen

atoms together with only small amount of carbon impurities,

which could be easily introduced from carbon dioxide at

atmosphere or from solvents and reactants at the synthesis

step Consequently, the tungsten oxide nanorod film used as

a sensing element can be considered simply as the condensed

collectivity of randomly arranged bundles of monoclinic

WO2.72 nanorods with high porosity and little carbon

impu-rities

Gas-sensing measurements were carried out by placing a sensor sample in a small chamber with electrical feedthrough, and by blowing diluted analyte vapor over it with the flow rate of 500 ml/ min while monitoring the resis-tances of the sensing layer and the heater Figure 2 shows the variations of normalized resistances at four different opera-tion temperatures for air-diluted 100 ppm NH3 exposure The measurements were performed sequentially with de-creasing the temperature from 250 to 20 ° C

Upon exposure to ammonia gas, decrease in resistance was observed at the operation temperature of 150– 250 ° C 关see Fig 2共a兲兴 This is understandable because tungsten ox-ide sensors are known to behave as an n-type

semiconductor.6–8Current in n-type MOS sensors is carried

by conduction band electrons, and adsorbates formed by at-mospheric oxygen at a grain boundary capture the electron carriers Upon exposure to reducing chemicals such as am-monia, the arrested electrons are released by the reactions between the reducing gas and the negatively charged oxygen adsorbates, leading to the decrease in resistance Completely opposite behavior, however, was observed for gas sensing at below 70 ° C; the resistance of the sensor film increases upon exposure to ammonia gas as displayed in Figs 2共c兲 and 2共d兲 The sensor response at 100 ° C shows a complex pattern 关Fig 2共b兲兴; rapid increase in resistance for NH3 injection, slow resistance decrease for duration of NH3pulsing, and the rapid decrease followed by slow increase for recovery pe-riod This variation can be understood by summing up the positive 共abnormal兲 and the negative 共ordinary兲 responses observed at low and high operation temperatures, respec-tively This kind of temperature-dependent response reversal was also observed for the reducing ethanol analyte

Further experiments were performed under different at-mosphere conditions for various analytes at room tempera-ture in order to clarify the gas sensing characteristics of our

WO2.72nanorod sensor Figures 3共a兲 and 3共b兲 show the sen-sor responses for 2% N2共or air兲, 1000 ppm ethanol, 10 ppm

NH3, and 3 ppm NO2 exposures in dry air and nitrogen

at-FIG 1 共a兲 Cross-sectional schematic diagram for well structure of a sensor

substrate with a membrane-based hotplate, 共b兲 plane-view

optical-microscope image of a fabricated sensor equipped with interdigitated

detec-tion electrodes, a microheater, and a sensing film, and 共c兲 surface SEM

image of the sensing layer deposited by drop coating of WO 2.72

measurement time at the operation temperature of 共a兲 200, 共b兲 100, 共c兲 70, and 共d兲 20 °C when tungsten oxide nanorod sensors are exposed to 100 ppm

NH 3

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mosphere, respectively All of them display the positive

re-sponse of increase in resistance except for the case of N2

injection with no discernable change, irrespective of

reduc-ing and oxidizreduc-ing analyte gases Their response magnitudes

ascend in the order of air, C2H5OH, NH3, and NO2

expo-sures, which seems to be correlated with the interaction

strength between the analyte vapors and the sensing layer

Since the desorption rate of adsorbed analytes greatly

de-pends on the bound interaction energy, the recovery time

becomes longer in the case of having the stronger interaction

strength, namely, the larger response The recovery process

was found to be accelerated by heating the sensing materials

at around 100 ° C for short duration共⬍60 s兲 or by

illuminat-ing 365-nm-UV light for about 10 s These phenomena are

probably due to the activated desorption process of adsorbent

species by imparted thermal or photon energy, as previously

reported in other 1D nanostructural MOS systems.2,12In

ad-dition, other volatile organic compounds such as toluene,

n-heptane, and acetone were also possible to detect at low

concentration less than 5 ppm at room temperature

For porous WO2.72-based sensors, the highly sensitive

increase in resistance under ambient conditions must be

mainly caused by the competition adsorption between

ambi-ent molecular oxygen and analyte vapors on the surface of

the active layer The molecular oxygen had been observed

predominantly as initial adsorbates instead of the ionosorbed

moieties at below 150 ° C.13 The nonstoichiometric WO2.72

films should have more favorable absorption sites due to its

oxygen-deficient defect structure than the stoichiometric

WO3 with several active sites.14 We believe that the high

sensitivity is attributed to the very small grain size and high

surface-to-volume ratios associated with the WO2.72nanorod

structures These nanostructural features allow the sensors to

be operated in the most sensitive, grain-controlled mode

hav-ing the completely depleted space charge region.15However,

the abnormal resistance increase upon exposure to reducing

analytes cannot be explained with the conventional space

charge model One of the conceivable mechanisms is the adverse effect of adsorbed analytes on the mobility of free charge carriers The number of collision experienced by the carriers in the bulk of the grain becomes comparable with the number of surface collisions because of the comparable di-mension between the mean free path of the carriers and the very thin nanorod thickness The adsorbates may function as active scattering centers, thus suppressing the electrical con-duction of free carriers, i.e., resulting in the resistance in-crease for both oxidizing and reducing analyte exposures

In conclusion, we have fabricated tungsten oxide gas sensors by using highly crystalline, tiny WO2.72 nanorods They show highly sensitive sensing ability for various reduc-ing and oxidizreduc-ing analytes even at room temperature When reducing gases exposure, the temperatudependent re-sponse was reversed from decrease in the sensor resistance at higher temperature to resistance increase at lower

tempera-ture Such unusual behavior, unprecedented for n-type MOS,

might be due to the unique structural features of nonstoichio-metric WO2.72 nanorod-based films with a high surface-to-volume ratio and active adsorption sites The facile vapor detection of WO2.72nanorod sensor at ambient temperatures might be successfully employed for the miniaturized sensing system, fulfilling the requirement of low power consumption This work was supported in part by the NRL and basic research programs of the ETRI and in part by the national research program for the 0.1 Terabit Non-volatile Memory Development sponsored by Korea Ministry of Commerce, Industry and Energy KL thanks the Korea Research Founda-tion Grant共KRF-2004–003–C00116兲

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FIG 3 Sensor responses for 2% N 2 共or air兲, 1000 ppm ethanol, 10 ppm

NH3, and 3 ppm NO2exposures in 共a兲 dry air and 共b兲 nitrogen atmosphere

at room temperature.

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